Lithium microbatteries can be used in various technological fields. A microbattery is produced by stacking a positive electrode material, an electrolytic material and a negative electrode material in thin layers. A microbattery is charged and discharged respectively with a transfer of lithium ions from one electrode to the other through the electrolyte, while electrons circulate via the external circuit. An improvement in the ionic conductivity of the electrolyte brings about a reduction in the internal resistance of the battery which can then be charged or discharged at a high current without an unacceptable ohmic drop.
Electrochromic systems can be formed by depositing an electrochemical cell, similar to that which forms a microbattery, onto a substrate made of glass or of transparent polymer covered with a transparent conducting layer. An electrochromic system is transparent and colorless in the charged state and colored in the discharged state or vice versa. Its operation relies on a transfer of ions from one electrode to another, as rapidly as possible, so as to minimize the response time of the system, said rapid transfer being promoted by an electrolyte having a high conductivity.
It is known to prepare electrolytes in thin film form by cathode sputtering. In this type of method, a target is used to obtain an electrolyte film made of amorphous material, it being understood that the amorphous structure generally enables the ionic conductivity of the electrolyte to be improved.
U.S. Pat. No. 5,512,147 and U.S. Pat. No. 5,597,660 describe the preparation and use of a thin film of LixPOyNz (with x around 2.8, 2y+3z≈7.8 and 0.16≦z≦0.46) as a solid electrolyte in a microbattery. The film is prepared by cathode sputtering of an Li3PO4 target in an atmosphere of nitrogen. The ionic conductivity of the film is around 2×10−6 S/cm.
K. H. Joo, P. Vinatier et al., [Solid State Ionics, 2003, Vol. 160, No. 1-2, pp. 51-59] describe electrolytes formed from a thin film (1-2 μm) of (1−X)LiBO2-xLi2SO4, x being 0.4 to 0.8. The maximum conductivity is obtained for x=0.7 and is 2.5×10−6 S/cm.
EP-1 365 470 describes an electrolyte formed from LIPS containing, in at %, 20% to 60% Li, 3% to 20% P, 30% to 60% S and optionally up to 5% O and/or N. The film forming the electrolyte can be deposited on a substrate that will serve as an electrode. Deposition can be carried out in particular by cathode sputtering. In an electrochemical cell that comprises an anode formed of a 10 μm layer of lithium, a thin layer (0.5 μm) of electrolyte obtained from a “78% Li2S−21.5 P2S5−0.5 Li3PO4” target, the ionic conductivity is 1.3×10−3 S/cm at 25° C. (Cf. p. 6, §42 and 44). The target used for cathode sputtering is a target consisting of sulfides sensitive to atmospheric moisture. Such a target is therefore brittle and unsuited to use on an industrial scale.
S. J. Lee et al., [Electrochem. Commun., 2003, Vol. 5, No. 1, pp. 32-35] describe a battery in which the electrolyte is Li1.9Si0.28P1.0O1.1N1.0, the active material of the positive electrode is LiCoO2 and the negative electrode is Si0.7V0.3. The conductivities obtained for the electrolyte are around 8.8×10−6 S/cm, a value that is quite close to those obtained in the present invention. The thin film of LiSiPON is obtained by cathode sputtering using a target consisting of Li3PO4 and Li2SiO3.